Barriers to internal rotation in thioamides. Experimental results and molecular orbital calculations

1967 ◽  
Vol 71 (7) ◽  
pp. 2318-2325 ◽  
Author(s):  
Jan Sandstrom
Keyword(s):  
Internal Rotation ◽  
Molecular Orbital ◽  
Experimental Results ◽  
Molecular Orbital Calculations ◽  
Barriers To Internal Rotation

Conformational preferences and barriers to internal rotation in fluorothioanisoles from long-range spin–spin couplings, photoelectron spectroscopy, and semiempirical molecular orbital calculations

1993 ◽  
Vol 71 (10) ◽  
pp. 1741-1750 ◽  
Author(s):  
Dietmar Chmielewski ◽  
Nick Henry Werstiuk ◽  
Timothy A. Wildman
Keyword(s):  
Internal Rotation ◽  
Molecular Orbital ◽  
Long Range ◽  
Photoelectron Spectra ◽  
Semiempirical Methods ◽  
Molecular Orbital Calculations ◽  
Methyl Carbon ◽  
Conformational Preferences ◽  
Barriers To Internal Rotation ◽  
Semiempirical Molecular Orbital

The conformational preferences and barriers to internal rotation about the S—C(phenyl) bond have been investigated for thioanisole and its 2-fluoro, 2,6- and 3,5-difluoro, and 2,3,5,6-tetrafluoro derivatives. Measurements of long-range spin–spin couplings between the methyl carbon and the para ring proton indicate that the 2-fluoro and 3,5-difluoro compounds prefer conformations with all heavy atoms coplanar. The 2,6-difluoro and 2,3,5,6-tetrafluoro compounds prefer conformations in which the methyl carbon lies in or near the plane perpendicular to the aromatic ring. Semiempirical molecular orbital calculations with the MNDO method indicate that all of the molecules prefer perpendicular conformations while similar calculations with AM1 indicate that all prefer planar conformations. Apparently the conformational behaviour can be quite sensitive to subtle changes in intramolecular interactions, which may indicate improvements to these semiempirical methods. The NMR results have been used to derive an internally consistent set of rotational potentials. Synthetic photoelectron spectra derived from these potentials and the AM1 orbital energies are in good agreement with the experimental spectra.

Structural features of (C5H5)2Mo2S6 clusters: an EHMO study

1989 ◽  
Vol 67 (11) ◽  
pp. 1931-1935 ◽  
Author(s):  
Janice A. Cramer ◽  
Michael J. McGlinchey ◽  
Jean-Yves Saillard
Keyword(s):  
Molecular Orbital ◽  
Structural Features ◽  
Experimental Results ◽  
Planar Structure ◽  
Molecular Orbital Calculations ◽  
Sandwich Compounds ◽  
Extended Hückel

Extended Hückel molecular orbital calculations on S6 in the planar D6h configuration and in the bent C2v geometry reveal that the former is favored for S62+ and the latter for S64−. It is shown that the additional six electrons would populate one π* and two σ* orbitals in the planar structure. In contrast, when these hexasulfur fragments are incorporated in triple-decker sandwich compounds the pseudo-D6h structure is favored for [(C5H5)Mo(S6)Mo(C5H5)]6+ but the C2v geometry is predicted for [(C5H5)Mo(S6)Mo(C5H5)]2+ which possesses only four more electrons. The required extra two electrons are shown to originate from a metallic δ-type molecular orbital. These EHMO calculations are in complete accord with some very recent experimental results from Gillespie's laboratory. Keywords: Molybdenum–sulfur clusters, EHMO calculations.

The Electronic Absorption Spectra of Some N-Substituted Pyrroles—Molecular Orbital Calculations

1982 ◽  
Vol 36 (3) ◽  
pp. 297-301 ◽  
Author(s):  
R. Abu-Eittah ◽  
R. Hilal ◽  
M. S. El-Shall
Keyword(s):  
Absorption Spectra ◽  
Molecular Orbital ◽  
Electronic Absorption ◽  
Electronic Transition ◽  
Electronic Absorption Spectra ◽  
Experimental Results ◽  
Electronic Transitions ◽  
Molecular Orbital Calculations ◽  
Nonpolar Solvents ◽  
State Functions

The electronic absorption spectra of N-phenylpyrrole and some of its parasubstituted derivatives have been investigated in polar and nonpolar solvents. Such an investigation could predict the extent of resonance interaction between the pyrryl and phenyl moieties of the composite molecule. The direction and polarization of the electronic transitions were predicted. Molecular orbital calculations using the SCF-CI procedures were performed on N-phenylpyrrole and some of its p-substituted derivatives. The state functions and energies were computed. The weight of each configuration was evidence for the direction of the electronic transition. The correspondence between the theoretical and experimental results is satisfactory.

scholarly journals The para substituent dependence of the internal rotational barrier in benzenethiol

1977 ◽  
Vol 55 (3) ◽  
pp. 552-556 ◽  
Author(s):  
Ted Schaefer ◽  
William J. E. Parr
Keyword(s):  
Ab Initio ◽  
Internal Rotation ◽  
Molecular Orbital ◽  
Rotational Barrier ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Fluorine Nucleus ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

On the basis of the observed spin–spin coupling constants between the sulfhydryl and ring protons and a hindered rotor treatment of the twofold barrier to internal rotation in a series of para substituted benzenethiol derivatives, it is argued that V2 is essentially zero in p-amino-benzenethiol and is 2.5 ± 0.2 kcal/mol in p-nitrobenzenethiol; having intermediate values for the methoxy, fluoro, methyl, and bromo derivatives in solution. The results are based on an assumed relationship between the four-bond and the fictitious six-bond couplings to the sulfhydryl proton. The conclusions are consistent with the observed magnitudes of the couplings over six and seven bonds, respectively, between the sulfhydryl proton and the fluorine nucleus and the methyl protons in the appropriate derivatives; as well as with the coupling between the sulfhydryl and methyl protons in 4-bromo-3-methylbenzenethiol. The experimental barriers are compared with ab initio molecular orbital calculations of their substituent dependence.

An Ab Initio Molecular Orbital Study of Sulfur-Substituted Carbanions: Toward an Understanding of Regiochemistry in the Chlorination of Unsymmetrical Sulfides

1997 ◽  
Vol 50 (6) ◽  
pp. 517 ◽  
Author(s):  
Jack Leon Ginsburg ◽  
Katherine Valenta Darvesh ◽  
Patricia Axworthy ◽  
Richard Francis Langler
Keyword(s):  
Free Radicals ◽  
Ab Initio ◽  
Molecular Orbital ◽  
Dimethyl Sulfide ◽  
Experimental Results ◽  
Molecular Orbital Calculations ◽  
Molecular Orbital Study ◽  
Carbonium Ions ◽  
Moller Plesset ◽  
Better Than

Moller-Plesset molecular orbital calculations at the 6-31+G(d) level have been completed on substituted dimethyl sulfide-derived carbanions. The results are applied to the prediction of regiochemistry for chlorination of unsymmetrical sulfides. Notwithstanding the need for future improvements, regiochemical predictions based on stabilities of sulfur-substituted carbanions match experimental results better than earlier predictions based on stabilities of sulfur-substituted carbonium ions or sulfur-substituted free radicals. The present results suggest that the mechanism for the elimination step in the Pummerer rearrangement of a chlorosulfonium cation is an E 1cb-like process.

Ab initio studies on amides: acetamide, N-methylformamide and N-methylacetamide

1982 ◽  
Vol 35 (6) ◽  
pp. 1071 ◽  
Author(s):  
L Radom ◽  
NV Riggs
Keyword(s):  
Ab Initio ◽  
Internal Rotation ◽  
Molecular Orbital ◽  
Basis Sets ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
Methyl Group ◽  
Barriers To Internal Rotation ◽  
Reported Data

Ab initio molecular orbital theory with the STO-3G and 4-31G basis sets has been used to investigate the geometries, preferred conformations, and barriers to internal rotation for acetamide, N-methylformamide and N-methylacetamide. Results are compared with corresponding previously reported data for formamide. For acetamide, the preferred conformation has the methyl group staggered with respect to the N-C bond whereas for N-methylformamide the methyl group is eclipsed with respect to this bond. Both N-methylformamide and N-methylacetamide prefer a Z-arrangement, i.e. methyl cis to C=O about the N-C bond. Experimentally determined barriers to internal rotation about the N-C bond generally lie within the range spanned by the STO-3G and 4-31G estimates.

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